1. Field of the Disclosure
The disclosure herein relates generally to the use of resistivity tools for determination of properties of earth formations.
2. Background of the Art
Electromagnetic induction resistivity well logging instruments or tools are used to determine the electrical conductivity and its converse, resistivity, of earth formations penetrated by a borehole. Formation conductivity is determined based on results of measuring the magnetic field of eddy currents induced in the formation adjoining the borehole. The electrical conductivity is used for, among other things, inferring the properties of the formation fluid. Typically, low conductivity (high resistivity) is associated with hydrocarbon-bearing earth formations.
Multi-component induction measurement tools are often utilized to measure horizontal and vertical resistivities (or conductivities), relative dip angles, strike angles, sand and shale content and water saturation. In addition, multi-component measurements may be used for analysis of fractured earth formations that may also have anisotropic layers. Often the induction tool is eccentric in the borehole during logging of non-vertical sections of the borehole. When the tool is eccentric, the current induced in borehole fluid by the transmitted signals is asymmetric and thus interferes with the signals received by the receivers from the formation. In some cases, the effects of eccentricity on the measurements can be severe. Some multi-component induction tools utilize spaced electrodes on the tool to reduce the effects of eccentricity. Others tools apply multi-frequency focus techniques. The results, in certain cases, still may not be satisfactory.
The present disclosure provides improved apparatus and methods that reduce the effects of eccentricity on the resistivity measurements.